1. Field of the Invention
The present invention relates to a liquid ejection apparatus and an air bubble determination method, and more particularly, to ejection determination in a liquid ejection apparatus which forms an image, or the like, on a medium by ejecting liquid from a nozzle.
2. Description of the Related Art
Inkjet recording apparatuses which comprise an inkjet head having a plurality of nozzles and record images onto a medium by ejecting ink toward the medium from the inkjet head, are known.
In an inkjet recording apparatus, if there is an increase in the viscosity of the ink or infiltration of air bubbles into the inkjet head, or if dirt, paper dust, or other foreign matter adheres to the ink ejection surface, then the nozzles may become blocked and it is difficult to eject ink droplets. If the nozzle blockages occur, then dot omissions occur in the image formed on the medium, and this causes degradation of the image quality. Some inkjet recording apparatuses are composed in such a manner that the nozzle blockages are determined and a maintenance operation is carried out in respect of the nozzles thus determined to have blockages.
An embodiment of ejection abnormality determination in nozzles relating to the related art is described with respect to FIGS. 17A to 17D. In the ejection abnormality determination according to the related art, the pressure generated in the pressure chambers is determined by means of pressure sensors provided in the pressure chambers, and ejection abnormalities in the nozzles connected to the pressure chambers are judged on the basis of the magnitude correlation between a peak value Vp of the waveform of the generated pressure and a prescribed threshold value Vth.
FIG. 17A shows a sensor signal 300 obtained from a pressure sensor. As shown in FIG. 17A, the sensor signal 300 has a voltage (waveform) that is directly proportional to the pressure of the pressure chamber (pressure waveform), and a pressure abnormality in the corresponding pressure chamber is judged on the basis of the magnitude correlation between the peak value Vp of the sensor signal 300 and the predetermined threshold voltage Vth.
A composition is adopted in which, if the peak value Vp of the sensor signal 300 is greater than the threshold voltage Vth, then the pulse signal 302 is obtained as shown in FIG. 17B. Furthermore, if the pulse signal 302 is obtained, then it is judged that the pressure in the pressure chamber is normal and that the nozzle connected to the pressure chamber is in normal ejection state.
On the other hand, the sensor signal 304 shown in FIG. 17C has a peak value V′p that is smaller than the threshold voltage Vth, and therefore the pulse signal (represented by numerical sign “302” in FIG. 17B) is not obtained as shown in FIG. 17D. If the pulse signal is not obtained, then it is judged that the pressure abnormality has occurred in the corresponding pressure chamber, and hence it is judged that the nozzle connected to the pressure chamber is in ejection abnormality state. In other words, by setting the threshold voltage Vth to an appropriate value, it is possible to judge that the pressure abnormality in the pressure chamber occurs and therefore the nozzle connected to the pressure chamber suffering the pressure abnormality is in the ejection abnormality state.
Furthermore, systems have also been proposed in which maintenance processing is carried out for the nozzles at a constant time interval, without determining the ejection abnormalities in the nozzles. In the system which carries out maintenance processing for the nozzles before the occurrence of the ejection abnormality in this way, it is possible to prevent the occurrence of the ejection abnormalities, in advance, by setting the time interval for maintenance processing to a suitable time.
The invention disclosed in Japanese Patent Application Publication No. 10-114074 relates to an inkjet recording head comprising electrostrictive vibrating elements provided respectively in the ink flow channels of a plurality of nozzles, which ejects ink droplets by applying a drive voltage to the electrostrictive vibrating elements. The inkjet recording head further comprises an air bubble determination device which determines the presence or absence of air bubbles in the ink flow channels by determining whether the voltage occurring in the electrostrictive vibrating elements due to the volume change in the ink flow channels becomes an excess voltage (in other words, the value of the voltage occurring in the electrostrictive vibrating elements becomes the value of the drive voltage or more) or not constantly during a printing operation.
However, in the embodiments shown in FIGS. 17A to 17D, the pressure abnormalities in the pressure chambers are judged on the basis of one constant threshold voltage. Therefore, even if there is a pressure abnormality that does not reach a sufficient level to affect ejection, it is judged on the basis of the pressure abnormality that the ejection abnormality has occurred, and hence the maintenance processing is carried out for the corresponding nozzle.
Furthermore, in a composition in which maintenance processing is carried out at a constant time interval, an expensive timer circuit is required in order to manage the interval for the maintenance processing, and since the maintenance is carried out irrespectively of the presence or absence of the ejection abnormalities, then the ink consumption increases, which is not economical.
In the invention disclosed in Japanese Patent Application Publication No. 10-114074, the presence or absence of air bubbles is determined by determining whether the voltage occurring in the electrostrictive vibrating elements becomes an excess voltage, which is the drive voltage or more. Therefore, if there is even an air bubble of a small size which is not sufficient to affect ink droplet ejection, it is judged that the ejection abnormality has occurred. Therefore, unnecessary and wasteful restoration processing is carried out, and the ink consumption increases, which is not economical.